Why is Migration Necessary?

Although J2EE specifications broadly cover requirements for applications, they
are nonetheless evolving standards. They either do not cover some aspects of applications
or leave implementation details to the application providers.

This leads to different implementations of the application servers, also well
as difference in the deployment of J2EE components on application servers. The array
of available configuration and deployment tools for use with any particular application
server product also contributes to the product implementation differences.

The evolutionary nature of the specifications itself presents challenges to
application providers. Each of the component APIs are also evolving. This leads to
a varying degree of conformance by products. In particular, an emerging product, such
as the Application Server, has to contend with differences in J2EE application components,
modules, and files deployed on other established application server platforms. Such
differences require mappings between earlier implementation details of the J2EE standard,
such as file naming conventions, messaging syntax, and so forth.

Moreover, product providers usually bundle additional features and services
with their products. These features are available as custom JSP tags or proprietary
Java API libraries. Unfortunately, using these proprietary features renders these
applications non-portable.

What Needs to be Migrated

For migration purposes, the J2EE application consists of the following file
categories:

Deployment descriptors (XML files)

JSP source files that contain Proprietary APIs

Java source files that contain Proprietary APIs

Deployment descriptors (XML files)

Deployment is accomplished by specifying deployment descriptors (DDs) for standalone
enterprise beans (EJB JAR files), front-end Web components (WAR files) and enterprise
applications (EAR files). Deployment descriptors are used to resolve all external
dependencies of the J2EE components/applications. The J2EE specification for DDs is
common across all application server products. However, the specification leaves several
deployment aspects of components pertaining to an application dependent on product-implementation.

JSP source files

J2EE specifies how to extend JSP by adding extra custom tags. Product vendors
include some custom JSP extensions in their products, simplifying some tasks for developers.
However, usage of these proprietary custom tags results in non-portability of JSP
files. Additionally, JSP can invoke methods defined in other Java source files as
well. The JSPs containing proprietary APIs needs to be rewritten before they can be
migrated.

Java source files

The Java source files can be EJBs, servlets, or other helper classes. The EJBs
and servlets can invoke standard J2EE services directly. They can also invoke methods
defined in helper classes. Java source files are used to encode the business layer
of applications, such as EJBs.Vendors bundle several services and proprietary Java
API with their products. The use of proprietary Java APIs is a major source of non-portability
in applications. Since J2EE is an evolving standard, different products can support
different versions of J2EE component APIs. This is another aspect that migration addresses.

What is Deployment of Migrated Applications?

Deployment refers to deploying a migrated application that was previously deployed
on an earlier version of Sun’s Application Server, or any third party application
server platforms.